We have previously shown that the regulation of prolactin secretion by estrogen (E) and progesterone (P) is similar in guinea pigs and monkeys. In monkeys, we observed that E increased the mRNA for the rate limiting enzyme in serotonin synthesis, tryptophan hydroxylase (TPH), and that P had no further effect. In guinea pigs, we previously observed that E had no effect on serotonin but E+P significantly increased serotonin content in the raphe nucleus. Thus, we have a discrepancy between mRNA levels in monkeys and transmitter levels in guinea pigs. We are questioning whether the mRNA levels are directly reflected in TPH protein levels. As a first approach, the levels of TPH protein are being measured in the raphe of E- and E+P-treated guinea pigs using Western blotting and densitometric analysis. Our preliminary results indicate that E causes a modest increase in TPH protein, and that addition of P causes a significant increase in TPH protein. Hence, at this time, the level of TPH protein in guinea pig correlates best with the level of serotonin transmitter measured in guinea pig and not with the level of TPH mRNA measured in monkey. We will next examine the level of TPH protein in monkey and the level of TPH mRNA in guinea pig to determine if this is due to a difference between the species or if P is acting on post-transcriptional processes. That is, if TPH protein in the monkey is significantly increased with E+P (whereas the mRNA increased with E but P had no further effect on mRNA levels), then P is acting between the mRNA and the protein to enhance translation of the mRNA into protein. The possibility that P is acting on a post-transcriptional process is quite novel.